As the bright sun shines upon the water, the water slowly disappears. the same sunlight gives energy to the surrounding plants to convert water and carbon dioxide into sugar and oxygen gas. which change is physical and which change is chemical?

The physical change is when water slowly disappears when the sun shines upon it. Its weight and volume decreases.

The chemical changes is when the plants absorb the energy from the sunlight and convert water and carbon dioxide into sugar and oxygen gas. Physically, nothing has changed. The plant still looks the same. However, changes occur internally; these internal changes are chemical in nature. 

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature